The present invention relates to a mechanism for detecting a load, and more particularly to a linear movement type load detecting mechanism for use in a top loading scale.
In general, a condition required for the load detecting mechanism is to measure faithfully a force exerting only in a measuring direction. That is to say, when a force is applied perpendicularly to the measuring direction, the mechanism must resist it and a measured result should not be affected at all. Particularly, for the top loading scale even if a load is applied at an off-center position of a weighing pan, a so-called shift error should not occur.
FIG. 1 shows a typical example of a known load detecting mechanism for use in the top loading scale. In FIG. 1, on a top of a pan receiving rod 1 is placed a pan not shown. To the rod is applied a detecting load due to the gravitational force exerting in a Z axis. The pan receiving rod 1 is extended below as a load shaft 2 and its lower end 3 is connected to a load cell 5 placed on a base 4. The load cell 5 may be generally formed by a suitable load transducer having a small displacement such as strain gauges, electromagnetic force balance mechanism, tuning fork vibrator, vibrating string and gyroscope. On the base 4 there are secured two posts 6 and 7, and two V-shaped arms 8 and 9 constituting Roberval mechanisms are supported by the posts 6 , 7 and load shaft 2 one above the other in parallel with each other in such a manner that root portions 8a and 9a of arms are secured to the load shaft 2, and branch ends 8b, 8c and 9b, 9c of arms are secured to the posts 6 and 7. Between upper ends of the posts 6 and 7 is arranged a reinforcing bar 10. The V-shaped arms 8 and 9 are formed to have a sufficient stiffness, but near the root portions and front ends are formed flexures 11a, 11b, 11c, 12a, 12b, 12c serving as flexible hinges so that the arms can be relatively freely bent at these portions.
An operation of the known mechanism will be explained with reference to FIG. 2. The link mechanism forms a parallelogram consisting of four link members or apexes at the flexures 11a, 11b, 12a and 12b. Since the positions of the flexures 11b and 12b are selected as fixed points, the weighing pan and load shaft 2 secured to the movable flexures 11a and 12a can move only circularly as shown by double headed arrows. When a load W is applied on the pan 13 at a position deviated from the center of pan in an X direction, compression force and tension force are applied to the arms 8 and 9, respectively, but as long as the arms are in parallel with each other, even if they are not arranged precisely in horizon, vertical components due to the compression and tension forces are cancelled out each other, so that only the force exerting in the axial direction, i.e. the vertical direction can be transmitted to the load shaft 2.
However, the known detecting mechanism shown in FIG. 1 has the following disadvantages.
(a) The mechanism has a relatively small resistance against a force exerting horizontally to the pan receiving rod 1 and might respond differently to said force, and further the Roberval mechanism is liable to twist in response to a force exerting at the root portion 8a of arm 8 in a direction Y1 perpendicular to the longitudinal direction X. PA0 (b) When elongated hinges forming the flexures 11a to 11c, 12a to 12c are not in parallel with each other in directions Y1 and Y2, and Y1' and Y2', there might be produced the shift error due to the deviated load. PA0 (c) When the branch portions of arms 8 and 9 are not precisely in parallel with each other, there might be produced the shift error due to the load deviated mainly in the X direction. PA0 (d) Since the load shaft 2 is moved circularly, while it is remained vertically, the load shaft 2 moves not only in the vertical direction, but also in the horizontal direction to a small extent. Therefore, undesired horizontal force is applied to the load cell and thus a measuring error might occur. This results in that use could not be made of a load cell having a large displacement, and even if the load cell with the small displacement is used, between the load shaft 2 and load cell 5 there could not be provided a spring or damper having a large displacement for absorbing and mitigating shock and overload upon applying the load W on the pan.
In order to solve the above problems of the known technique, the inventor has proposed, in Japanese Patent Application Laid-open Publication No. 94,016/84, a load detecting mechanism in which a load shaft is moved linearly. However, this mechanism has another drawback that the load shaft is not only moved linearly, but also is rotated about its axis and that some parts thereof are difficult to manufacture.